1. Field of the Invention
The present invention relates to a differential circuit using a field effect type semiconductor element (FET), and particularly, it relates to a differential circuit mounted on a semiconductor integrated circuit.
2. Description of the Related Art
Hereinafter, the explanation of a differential circuit will be given, as an example, when a gallium arsenic MESFET (metal-semiconductor junction FET) is used.
The basic type of FET differential amplifier circuit is usually formed by transistors as a differential pair, and has load resistors connected between a high potential source and a low potential source.
But, in such a circuit, a sufficient voltage gain cannot be obtained because of the load resistors. Therefore, to improve the voltage gain, it has been proposed to replace the load elements and the current source with a constant current type FET in which a short-circuit is made between the gate and the source.
However, this type of circuit has the following drawbacks:
(1) When an input is high level a current flows into a gate of a driver FET, and if the value of this current is large, a break down of a wire or a deterioration of the Schottky characteristic may occur.
(2) The dependency of a circuit characteristic on an element characteristic is high, and thus the circuit characteristic is easily changed by a variation in the element characteristic.
To solve this problem, it has been proposed that both ends of the load FET be clamped by diodes, to obtain a stable operation.
However, this clamping by the diodes causes other problems, such as:
A large temperature characteristic at the low output level, which occurs because the low output level is determined by a forward ON voltage of the I-V characteristic of the diode; and a deterioration of the AC characteristic because the junction capacitance of the diode becomes a load.